By Jeff Mason and Susan Heavey WASHINGTON (Reuters) – White House COVID-19 coordinator Jeff Zients will leave his post next month and will be replaced by public health expert Dr. Ashish Jha, U.S. President Joe Biden said on Thursday, as the administration prepares for new variants that could hit the country. Jha, an internist who leads the Brown University School of Public Health and is a television commentator, takes on the new role as the United States shifts to a new phase of the pandemic two years after the novel coronavirus upended the nation. “Americans are safely moving back to more nor…